swift: issue in converting string to double

There are two different issues here. First – as already mentioned in
the comments – a binary floating point number cannot represent the
number 8.7 precisely. Swift uses the IEEE 754 standard for representing
single- and double-precision floating point numbers, and if you assign

let x = 8.7

then the closest representable number is stored in x, and that is

8.699999999999999289457264239899814128875732421875

Much more information about this can be found in the excellent
Q&A Is floating point math broken?.

---

The second issue is: Why is the number sometimes printed as “8.7”
and sometimes as “8.6999999999999993”?

let str = "8.7"
print(Double(str)) // Optional(8.6999999999999993)

let x = 8.7
print(x) // 8.7

Is Double("8.7") different from 8.7? Is one more precise than
the other?

To answer these questions, we need to know how the print()
function works:

• If an argument conforms to CustomStringConvertible, the print function calls its description property and prints the result
  to the standard output.
• Otherwise, if an argument conforms to CustomDebugStringConvertible,
  the print function calls is debugDescription property and prints
  the result to the standard output.
• Otherwise, some other mechanism is used. (Not imported here for our
  purpose.)

The Double type conforms to CustomStringConvertible, therefore

let x = 8.7
print(x) // 8.7

produces the same output as

let x = 8.7
print(x.description) // 8.7

But what happens in

let str = "8.7"
print(Double(str)) // Optional(8.6999999999999993)

Double(str) is an optional, and struct Optional does not
conform to CustomStringConvertible, but to
CustomDebugStringConvertible. Therefore the print function calls
the debugDescription property of Optional, which in turn
calls the debugDescription of the underlying Double.
Therefore – apart from being an optional – the number output is
the same as in

let x = 8.7
print(x.debugDescription) // 8.6999999999999993

But what is the difference between description and debugDescription
for floating point values? From the Swift source code one can see
that both ultimately call the swift_floatingPointToString
function in Stubs.cpp, with the Debug parameter set to false and true, respectively.
This controls the precision of the number to string conversion:

  int Precision = std::numeric_limits<T>::digits10;
  if (Debug) {
    Precision = std::numeric_limits<T>::max_digits10;
  }

For the meaning of those constants, see http://en.cppreference.com/w/cpp/types/numeric_limits:

• digits10 – number of decimal digits that can be represented without change,
• max_digits10 – number of decimal digits necessary to differentiate all values of this type.

So description creates a string with less decimal digits. That
string can be converted to a Double and back to a string giving
the same result.
debugDescription creates a string with more decimal digits, so that
any two different floating point values will produce a different output.

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Summary:

• Most decimal numbers cannot be represented exactly as a binary
  floating point value.
• The description and debugDescription methods of the floating
  point types use a different precision for the conversion to a
  string. As a consequence,
• printing an optional floating point value uses a different precision for the conversion than printing a non-optional value.

Therefore in your case, you probably want to unwrap the optional
before printing it:

let str = "8.7"
if let d = Double(str) {
    print(d) // 8.7
}

For better control, use NSNumberFormatter or formatted
printing with the %.<precision>f format.

Another option can be to use (NS)DecimalNumber instead of Double
(e.g. for currency amounts), see e.g. Round Issue in swift.